Mass spectrometry is a powerful tool for identifying analytes in a sample. Applications are legion and include identifying biomolecules, such as carbohydrates, nucleic acids and steroids, sequencing biopolymers such as proteins and saccharides, determining how drugs are used by the body, performing forensic analyses, analyzing environmental pollutants, and determining the age and origins of specimens in geochemistry and archaeology.
In mass spectrometry, a portion of a sample is transformed into a gas containing analyte ions. The gaseous analyte ions are separated in the mass spectrometer according to their mass-to-charge (m/z) ratios and then detected by a detector. In the detector, the ion flux is converted to a proportional electrical current. The mass spectrometer records the magnitude of these electrical signals as a function of m/z and converts this information into a mass spectrum that can be used to identify the analyte.
For example, in quadrupole mass spectrometers, a time-dependent electric field, which is generated by applying appropriate voltages to an arrangement of conductors, exerts forces on ions near the conductors. The trajectories of the ions depend on their m/z ratio. By choosing appropriate voltages, ions injected in the space between the conductors having m/z values that fall in a small interval centered about a particular m/z are transmitted and then detected by a detector. Other ions having m/z values falling outside this interval are filtered out without being detected.
One common arrangement of electrodes is that of a quadrupole spectrometer comprising four parallel rods and two end devices, such as end plates or lenses. Various voltages can be applied to the rods and end plates. For example, both pairs of rods can be subjected to an RF voltage and a DC voltage (RF/DC mass spectrometer), or both pairs of rods can be subjected to only an RF voltage (RF-only mass spectrometer). Applying a DC voltage to the end plates traps the ions, before a portion are ejected for detection (ion trap mass spectrometer). Similar systems can also be used as ion guides. In addition to trapping ions in ion trap mass spectrometers, the end plates also generally serve to terminate the fields arising from the quadrupole rods.
The electric field of an ideal arrangement of infinitely long rods in the absence of end plates yields a relatively simple electrical field. In particular, when the four rods are disposed on the edges of a box and RF fields are applied to the rods so that opposite edges are in phase and adjacent edges are out of phase by 180°, a quadrupolar field arises. However, the finite length of the rods and the presence of the end plates in laboratory mass spectrometers give rise to non-ideal behavior. In particular, penetration of the end fields into the axial region of the quadrupole rods causes a local distortion of the ideal quadrupolar field and gives rise to a fringing field that is most prominent near the entrance plate and the exit plate.
Thus, in a multipole mass spectrometer or ion guide, ions in the vicinity of the end plates experience fields that are not entirely quadrupolar, due to the nature of the termination of the main RF and DC fields near the entrance and exit plates. Fringing fields couple the radial and axial degrees of freedom of the trapped ions. In contrast, near the center of the rod arrangement, further removed from the end plates and fringing fields, the axial and radial components of ion motion are not coupled or are minimally coupled.
The fringing fields couple the radial and axial degrees of freedom of the trapped ions. In certain ion trap mass spectrometers, this fact can be exploited to eject ions axially, as described in U.S. Pat. No. 6,177,668, the contents of which are herein incorporated by reference. In particular, in a quadrupolar rod configuration with end plates, ions can be trapped, and then, by scanning the frequency of a low voltage auxiliary AC field, ions of a particular m/z value can be axially ejected out of the trap for detection.
The auxiliary AC field is an addition to the trapping DC voltage supplied to end plates and couples to both radial and axial secular ion motion. The auxiliary AC field is found to excite the ions sufficiently that they surmount the axial DC potential barrier at the exit plate, so that they can leave axially. The deviations in the field in the vicinity of the exit plate leads to the above-described coupling of axial and radial ion motions. This coupling enables the axial ejection of ions at radial secular frequencies, which ions may then be analyzed according to the usual techniques of mass spectrometry. In contrast, in a conventional ion trap, excitation of radial secular motion generally leads to radial ejection, and excitation of axial secular motion generally leads to axial ejection.
This use of the fringing fields to axially eject ions from ion traps for mass analysis, as well as the role of these fields in RF/DC and RF-only mass spectrometers, underscores the importance of understanding and controlling the fringing fields.
These fringing fields play a large role in the performance of multipole mass spectrometers. Entrance fringing fields can significantly change the ion acceptance properties of RF/DC quadrupole mass spectrometers and these fringing fields have been studied by several investigators.
Exit fringing fields have been shown to be important for operation of RF-only quadrupole mass spectrometers as well as linear ion trap mass spectrometers with axial ion ejection. In these devices the mechanism of action is intimately tied to the radial-to-axial coupling of the ion motion induced in the exit fringing field region of the multipole.